1. Field of the Invention
This invention relates to a system for driving a gas discharge lamp and more specifically to a system employing pulse width modulation for reducing color segregation in high intensity gas discharge lamps.
2. Description of Related Art
High intensity discharge lamps (HID) are becoming increasingly popular because of their many advantages, such as efficiency and light intensity. These HID lamps are driven by either a high frequency electronic ballast that is configured to generate driving current signals at above 20 KHz range or by a low frequency electronic ballast with driving current signals in the 100 Hz range.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at many instances, can cause flicker of the arc which is very annoying to humans. Furthermore, acoustic resonance can cause the discharge arc to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge lamp.
Recently, a new class of high intensity discharge lamps has been developed that employ ceramic (polycrystalline alumina) envelopes. The discharge envelope in this class of lamps is cylindrical in shape, and the aspect ratio, i.e., the inner length divided by the inner diameter is close to one, or in some instances more than one. Such lamps have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs.
The color segregation can be observed by projecting an image of the arc onto a screen, which shows that the bottom part of the arc appears pink, while the top part appears blue or green. This is caused by the absence of complete mixing of the metal additives in the discharge. In the upper part of the discharge there is excessive thallium emission and insufficient sodium emission. This phenomena leads to high color temperature and decreased efficacy.
U.S. Pat. No. 6,184,633 entitled Reduction of Vertical Segregation In a Discharge Lamp, incorporated herein by reference, teaches a method to eliminate or substantially reduce acoustic resonance and color segregation, by providing a current signal frequency sweep within a sweep time, in combination with an amplitude modulated signal having a frequency referred to as second longitudinal mode frequency. The typical parameters for such operation are a current frequency sweep from 45 to 55 kHz within a sweep time of 10 milliseconds, a constant amplitude modulation frequency of 24.5 KHz and a modulation index of 0.24. The modulation index is defined as (Vmaxxe2x88x92Vmin)/(Vmax+Vmin), where Vmax is the maximum peak to peak voltage of the amplitude modulated envelope and Vmin is the minimum peak to peak voltage of the amplitude modulated envelope. The frequency range of 45 kHz to 55 kHz is between the first azimuthal acoustic resonance mode and the first radial acoustic resonance mode. The second longitudinal mode can be derived mathematically, where the power frequency of the nth longitudinal mode is equal to n*C1/2L where xe2x80x9cnxe2x80x9d is the mode number, xe2x80x9cC1xe2x80x9d is the average speed of sound in the axial plane of the lamp and xe2x80x9cLxe2x80x9d is the inner length of the lamp.
What is needed is an electronic ballast that can provide power to high intensity discharge lamps that (i) substantially reduces color segregation, (ii) substantially reduces acoustic resonances/arc instabilities, (iii) can be used with either horizontally or vertically oriented high intensity discharge lamps, and (iv) can be implemented at relatively low cost.
In one aspect, the present invention is directed to a ballast system for driving a gas discharge lamp. In one embodiment, the ballast system comprises a ballast bridge circuit for generating a pulse voltage signal. The bridge circuit has an operational frequency, a power input for receiving power from a power source, and a control signal input for receiving a signal that effects sweeping of the operational frequency and generation of the pulse voltage signal. The system further comprises a controller for generating the control signal so as to control the bridge circuit. The controller comprises circuitry that generates a signal comprising a frequency swept signal that is amplitude modulated by a fixed frequency signal. This signal is inputted into the control signal input of the bridge circuit. The controller further comprises circuitry for periodically sweeping the operational frequency of the bridge circuit. The system also includes a filter circuit for filtering the pulse voltage signal. In one embodiment, the sweep of the operational frequency of the bridge circuit is a non-linear sweep.
In one embodiment, the filter is configured to reproduce the desired power frequency components below about 150 kHz and maintain the components of the power frequencies above about 150 kHz at least 7.8 dB below the amplitude of the component at 150 kHz. In another embodiment, the filter is configured to reproduce the desired power frequency components below about 150 kHz and maintain the components of the power frequencies above about 150 kHz at least 10.8 dB below the amplitude of the component at 150 kHz.
In another aspect, the present invention is directed to a method for driving a gas discharge lamp, comprising the steps of providing a ballast system having a bridge circuit for generating a pulse voltage signal wherein the bridge circuit has an operational frequency, a power input for connection to a power source, and a control signal input for receiving a signal that effects sweeping of the operational frequency and generation of the pulse voltage signal, generating a signal comprising a frequency swept signal that is amplitude modulated by a fixed frequency signal and inputting the generated signal into the control signal input of the bridge circuit, periodically sweeping the operational frequency of the bridge circuit, and filtering the pulse voltage signal generated by the bridge circuit.